An Introduction to JavaScript

transcript

INTRODUCTION TO WEB PROGRAMMING

Lecture 16: JavaScript

Copyright Notice

The materials (used by permission) in this presentation are from Web Programming Step by Step written by Marty Stepp, Jessica Miller, and Victoria Kirst. http://www.webstepbook.com/supplements-2ed.shtml

Document's original contents are © Copyright 2012 Marty Stepp, Jessica Miller, and Victoria; revised by Anthony Hetrick. All rights reserved. Any redistribution, reproduction, transmission, or storage of part or all of the contents in any form is prohibited without the author's expressed written permission.

JavaScript

1. Purposes of JavaScript2. Basic language structure and syntax3. Event-Driven Programming with JavaScript

Client-side scripting

client-side script: code runs in browser after page is sent back from server often this code manipulates the page or

responds to user actions

Why use client-side programming?

PHP already allows us to create dynamic web pages. Why also use client-side scripting?

client-side scripting (JavaScript) benefits: usability: can modify a page without having to post back to

the server (faster UI) efficiency: can make small, quick changes to page without

waiting for server event-driven: can respond to user actions like clicks and

key presses server-side programming (PHP) benefits:

security: has access to server's private data; client can't see source code

compatibility: not subject to browser compatibility issues power: can write files, open connections to servers, connect

to databases, ...

What is JavaScript?

a lightweight programming language ("scripting language")

used to make web pages interactive insert dynamic text into HTML (ex: user name) react to events (ex: page load user click) get information about a user's computer (ex: browser

type) perform calculations on user's computer (ex: form

validation) a web standard (but not supported identically by

all browsers) NOT related to Java other than by name and some

syntactic similarities

JavaScript vs. Java

interpreted, not compiled more relaxed syntax and rules

fewer and "looser" data types variables don't need to be declared errors often silent (few exceptions)

key construct is the function rather than the class "first-class" functions are used in many

situations contained within a web page and

integrates with its HTML/CSS content

JavaScript vs. PHP

similarities: both are interpreted, not compiled both are relaxed about syntax, rules, and types both are case-sensitive both have built-in regular expressions for powerful text

processing differences:

JS is more object-oriented: noun.verb(), less procedural: verb(noun)

JS focuses on UIs and interacting with a document; PHP on HTML output and files/forms

JS code runs on the client's browser; PHP code runs on the web server

JavaScript

alert("message");

alert("Hello, I'm an alert message!");

A JavaScript statement: alert

a JS command that pops up a dialog box with a message

var name = expression;

var name = expression;var age = 32;var weight = 127.4;var clientName = "Connie Client";

Variables and types

variables are declared with the var keyword (case sensitive)

types are not specified, but JS does have types ("loosely typed") Number, Boolean, String, Array, Object, Function,

Null, Undefined can find out a variable's type by calling typeof

var enrollment = 99;var medianGrade = 2.8;var credits = 5 + 4 + (2 * 3);

Number type

integers and real numbers are the same type (no int vs. double)

same operators: + - * / % ++ -- = += -= *= /= %= similar precedence to Java many operators auto-convert types: "2" * 3 is 6

var s = "Connie Client";var fName = s.substring(0, s.indexOf(" ")); // "Connie"var len = s.length; // 13var s2 = 'Melvin Merchant'; // can use "" or ' '

String type

methods: charAt, charCodeAt, fromCharCode, indexOf, lastIndexOf, replace, split, substring,toLowerCase, toUpperCase charAt returns a one-letter String (there is no char type)

length property (not a method as in Java) concatenation with + : 1 + 1 is 2, but "1" + 1 is "11"

var count = 10;var s1 = "" + count; // "10"var s2 = count + " bananas, ah ah ah!"; // "10 bananas, ah ah ah!"var n1 = parseInt("42 is the answer"); // 42var n2 = parseFloat("booyah"); // NaN

More about String

escape sequences behave as in Java: \' \" \& \n \t \\

to convert between numbers and Strings:

var firstLetter = s[0];var firstLetter = s.charAt(0);var lastLetter = s.charAt(s.length - 1);

More about String

to access characters of a String, use [index] or charAt:

// single-line comment /* multi-line comment */

Comments (same as Java)

identical to Java's comment syntax recall: 4 comment syntaxes

HTML:  CSS/JS/PHP: /* comment */ Java/JS/PHP: // comment PHP: # comment

for (initialization; condition; update) { statements;}

for loop (same as Java)

var s1 = "hello";var s2 = "";for (var i = 0; i < s.length; i++) { s2 += s1[i] + s1[i];}// s2 stores "hheelllloo"

var sum = 0;for (var i = 0; i < 100; i++) { sum = sum + i;}

var rand1to10 = Math.floor(Math.random() * 10 + 1);var three = Math.floor(Math.PI);

Math object

methods: abs, ceil, cos, floor, log, max, min, pow, random, round, sin, sqrt, tan

properties: E, PI

Logical operators

> < >= <= && || ! == != === !== most logical operators automatically

convert types: 5 < "7" is true 42 == 42.0 is true "5.0" == 5 is true

=== and !== are strict equality tests; checks both type and value "5.0" === 5 is false

if (condition) { statements;} else if (condition) { statements;} else { statements;}

if/else statement (same as Java)

identical structure to Java's if/else statement

JavaScript allows almost anything as a condition

var iLike190M = true;var ieIsGood = "IE6" > 0; // falseif ("web dev is great") { /* true */ }if (0) { /* false */ }

Boolean type

any value can be used as a Boolean, "falsey" values: 0, 0.0, NaN, "", null, and undefined

"truthy" values: anything else converting a value into a Boolean explicitly:

var boolValue = Boolean(otherValue); var boolValue = !!(otherValue);

while (condition) { statements;}

do { statements;} while (condition)

while loops (same as Java)

break and continue keywords also behave as in Java

var name = []; // empty arrayvar name = [value, value, ..., value]; // pre-filledname[index] = value; // store element

var ducks = ["Huey", "Dewey", "Louie"]; var stooges = []; // stooges.length is 0stooges[0] = "Larry"; // stooges.length is 1stooges[1] = "Moe"; // stooges.length is 2stooges[4] = "Curly"; // stooges.length is 5stooges[4] = "Shemp"; // stooges.length is 5

Arrays

two ways to initialize an array length property (grows as needed when elements

are added)

var a = ["Stef", "Jason"]; // Stef, Jasona.push("Brian"); // Stef, Jason, Briana.unshift("Kelly"); // Kelly, Stef, Jason, Briana.pop(); // Kelly, Stef, Jasona.shift(); // Stef, Jasona.sort(); // Jason, Stef

Array methods

array serves as many data structures: list, queue, stack, ...

methods: concat, join, pop, push, reverse, shift, slice, sort, splice, toString, unshift push and pop add / remove from back unshift and shift add / remove from front shift and pop return the element that is removed

var s = "the quick brown fox";var a = s.split(" "); // ["the", "quick", "brown", "fox"]a.reverse(); // ["fox", "brown", "quick", "the"]s = a.join("!"); // "fox!brown!quick!the"

Splitting strings: split and join

split breaks apart a string into an array using a delimiter can also be used with regular

expressions surrounded by /:var a = s.split(/[ \t]+/);

join merges an array into a single string, placing a delimiter between them

function myFunction() { alert("Hello!"); alert("How are you?");}

function name() { statement ; statement ; ... statement ;}

Defining functions

the above could be the contents of example.js linked to our HTML page

statements placed into functions can be evaluated in response to user events

var ned = null;var benson = 9;var caroline; // at this point in the code,// ned is null// benson's 9// caroline is undefined

Special values: null and undefined

undefined : has not been declared, does not exist null : exists, but was specifically assigned an

empty or null value Why does JavaScript have both of these?

JavaScript

Linking to a JavaScript file: script

script tag should be placed in HTML page's head

script code is stored in a separate .js file JS code can be placed directly in the HTML

file's body or head (like CSS) but this is bad style (should separate content,

presentation, and behavior)

Event-driven programming

JS programs have no main; they respond to user actions called events

event-driven programming: writing programs driven by user events

<button>Click me!</button>

Buttons: <button>

button's text appears inside tag; can also contain images

To make a responsive button or other UI control: choose the control (e.g. button) and event (e.g.

mouse click) of interest write a JavaScript function to run when the event

occurs attach the function to the event on the control

<element attributes onclick="function();">...

<button onclick="myFunction();">Click me!</button>

Event handlers

JavaScript functions can be set as event handlers when you interact with the element, the function will

execute onclick is just one of many event HTML attributes

we'll use

An Introduction to JavaScript

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